1. Field of the Invention
The present invention relates to a full-color liquid crystal display device and a fabrication process therefor. More particularly, the invention relates to a full-color liquid crystal display device suitable for OA (office automation) systems such as word-processor, notebook personal computer and portable information terminal, various visual systems, game systems and the like, and to a fabrication process therefor.
2. Description of Related Arts
In recent years, rapid developments have been made in applications of liquid crystal display devices to word-processors, notebook type personal computers, portable information terminals, portable TVs and the like. Transmissive liquid crystal display elements such as of twisted nematic (TN) type and super-twisted nematic (STN) type are conventionally used for the liquid crystal display devices.
The TN-type liquid crystal display elements, which are disposed between a pair of polarizers, achieve monochromic display by utilizing optical characteristics for illumination from a back light disposed behind the display elements. More specifically, the TN-type liquid crystal display elements utilize optical switching characteristics attributable to the optical rotation of a liquid crystal which emerges with no voltage being applied and is negated with a voltage being applied. For color display, each pixel is divided into three sub-elements which are respectively provided with red-, green- and blue-color filters each having minute dimensions. By utilizing the aforesaid optical switching characteristics, multi-color or full-color display is achieved by way of the additive color process. At present, this principle is widely used in color liquid crystal display devices driven by active-matrix addressing or simple-matrix addressing.
Liquid crystal display devices utilizing the additive color process for color display, however, suffer from a low use efficiency of light (typically, several percent), thereby requiring a high-power back light. This is because the pixels thereof, which are each divided into three sub-elements respectively provided with red-, green- and blue- color filters, can utilize only one third the illumination passing through the liquid crystal layers thereof. Where such a liquid crystal display device is employed in a portable information terminal, a portable TV or a like system, power consumed by the back light accounts for the majority of the overall power consumption of the system. Therefore, the liquid crystal display device cannot enjoy the advantage of low power consumption of the liquid crystal display elements.
From the view point of energy saving and life-time extension of the portable information terminal or a like system, it is indispensable for the liquid crystal display devices to satisfy the low-power-consumption requirements.
To overcome this drawback, there are proposed a reflective liquid crystal display device employing no back light and a display method offering an improved use efficiency of light.
The reflective liquid crystal display device is disclosed in "Next-Generation Liquid Crystal Display Technology", pp. 167-196 (authored by Tatsuo Uchida and published by Kogyo Chosakai) of Japan. The reflective liquid crystal display device is adapted for full-color display, and has three liquid crystal panels respectively including liquid crystal layers containing cyan, magenta and yellow dyes and stacked one on another. These liquid crystal panels are independently driven to achieve full-color display by way of the subtractive color process.
According to the color display method by the subtractive color process, each pixel can utilize all illumination passing through the liquid crystal layers to produce multiple colors for full-color display. Therefore, the use efficiency of light is increased three times that of the full-color display method utilizing the additive color process.
However, the reflective liquid crystal display device suffers from color offset when viewed on the skew. This is because transparent glass substrates are disposed between the respective liquid crystal layers. Therefore, it is necessary to sufficiently reduce the thickness of the substrate relative to the size of each pixel. On the other hand, to independently drive the respective liquid crystal panels, a liquid crystal driving active element should be formed in each of the liquid crystal panels. Therefore, the substrates are required to have higher strength and heat-resistance. Where thin plastic plates are to be used as the substrates, for example, low strength and heat-resistance of the thin plastic plates may pose a difficult problem during a process for fabricating the liquid crystal panels. Further, the liquid crystal panels each require to be formed with a liquid crystal driving active element and a driver circuit, so that the fabrication of the reflective liquid crystal display device requires a complicated and expensive process.
The method for improving the use efficiency of light involves stacking of liquid crystal layers. Japanese Unexamined Patent Publications No. Sho 60(1985)-173520 and No. Sho 61(1986)-134789, for example, disclose guest-host liquid crystal cells having two liquid crystal layers disposed between three substrates. The two liquid crystal layers are driven by an active element formed on one of the three substrates. According to this method, the two guest-host liquid crystal layers are stacked such that the alignment of liquid crystal molecules in one liquid crystal layer is perpendicular to that in the other liquid crystal layer and, therefore, all the light components of natural light can be efficiently utilized.
However, the driving of the two liquid crystal layers is enabled by the single active element, making it impossible to independently drive the respective liquid crystal layers. Therefore, the liquid crystal cells cannot utilize the subtractive color process but the additive color process for color display. This offsets the advantage of full utilization of natural light.